Consider using pybind11 in favour of Boost.Python

[sindre-nistad]

pybind11 seem to be able to do what we are currently using Boost.Python for today.

Some of the reasons in favour of migrating

• Boost is overkill for our purposes
• compiling it, as part of the compilation of the wheels, has been a source of a few problems
• the abridged source code for Boost takes a significant part of our source distribution (~36 MB uncompressed)
• cmake/boost.cmake contributes quite a bit to the complexity of compiling gaussianfft
• The vast majority of what Boost has to offer is not used in our case

Some reasons against migrating

• Currently, the compilation, and everything else works as expected

[oddvarlia]

When using pybind11 instead of boost we avoid the issue which arise when boost version in RMS and boost version in gaussianfft are different.